sph-lib includes a multipart-form-data parser that can do partial reading and supports unlimited nesting of parts.

(import (sph) (sph web html))

(define example-data
  (open-input-string
    "-----------------------------19737173311706079401624261242\r\nContent-Disposition: form-data; name=\"content\"\r\n\r\naeu\r\n-----------------------------19737173311706079401624261242\r\nContent-Disposition: form-data; name=\"names\"\r\n\r\n[\"a\",\"b\"]\r\n-----------------------------19737173311706079401624261242\r\nContent-Disposition: form-data; name=\"type\"\r\n\r\ndocl\r\n-----------------------------19737173311706079401624261242--\r\n"))

(html-read-multipart-form-data example-data)

the result:

(((("Content-Disposition"
  ("form-data" . #t)
    ("name" . "type")))
  .
  "docl")
  ((("Content-Disposition"
    ("form-data" . #t)
    ("name" . "names")))
  .
  "[\"a\",\"b\"]")
  ((("Content-Disposition"
    ("form-data" . #t)
    ("name" . "content")))
  .
  "aeu"))

(html-multipart-form-data-ref result "submit-name")

((("Content-Disposition"
  ("form-data" . #t)
  ("name" . "submit-name")))
  .
  "Larry")